Some radio frequency (RF) transmitters include self-sensing circuits configured to measure the power and/or the phase of transmit signals. For example, RF transmitters for 77 gigahertz (GHz) automotive radar systems often include self-sensing circuits. The power measurements are important to ensure that the RF transmitters do not violate rules and regulations regarding maximum transmit power. The phase measurements are important for multiple-input and multiple-output (MIMO) applications of the RF transmitters.
One type of self-sensing circuit comprises separate components for power and phase. A diode averages an RF signal that is transmit and generates a direct current (DC) voltage that is proportional to the power of the RF signal. The RF signal is generated from a local oscillator (LO) signal, such that the RF signal has the same frequency as the LO signal. A double-sideband mixer receives the RF signal and the LO signal, and generates an intermediate frequency (IF) signal with a DC voltage that is proportional to the phase shift between the RF signal and the LO signal. A challenge with the self-sensing circuit is phase ambiguity since the DC voltage of the IF signal may correspond to two different phases. For example, the DC voltage may correspond to a phase in a left or right hemisphere of a polar coordinate system.
One solution to address the phase ambiguity is to employ an additional component, an IQ modulator, to shift the phase of the RF signal or the LO signal up and/or down before it passes to the double-sideband mixer. For example, the phase of the RF or LO signal may be shifted with the IQ modulator by adjusting the in-phase (I) signal of the IQ modulator and the quadrature (Q) signal of the IQ modulator. Depending on how the DC voltage of the IF signal changes in response to the phase shift, the phase between the RF signal and the LO signal can be determined without phase ambiguity.
While the self-sensing circuit provides power and phase measurements, it's dependent upon multiple components. For example, the self-sensing circuit has two components for power and phase measurement with phase ambiguity, and three components for power and phase measurements without phase ambiguity. The multiple components consume large amounts of area in integrated circuits (ICs) or on printed circuit boards (PCBs). Further, the multiple components lead to long testing and verification times, and also to high failure rates. Accordingly, the multiple components lead to high costs.